1. Field of the Invention
The present invention relates in general to a resistor unit, and more particularly to a resistor unit installed in a speed controller for controlling the speed of a fan motor employed in an automotive air conditioning device, and a method of producing the resistor unit. More specifically, the present invention is concerned with a resistor unit which generally comprises a resistor block including a flat resistor, a flat insulating plate and a flat heat radiation plate which are respectively positioned against one another, a holder block of molded plastic having the resistor block mounted thereon, a plurality of terminals partially embedded in the holder block and connected to given portions of the flat resistor, and coupling means for coupling the resistor block with the holder block.
2. Description of the Prior Art
A blower unit of an automotive air conditioning device has a blower installed therein. The blower has a fan held in a fan scroll and driven by an electric motor. The speed of the motor is controlled by a speed controller. The speed controller has a resistor unit including a plurality of resistors connected in series. By varying the total resistance of the resistor unit, the voltage applied to the electric motor is varied, so that the rotation speed of the fan can be controlled to, for example, a high level, a neutral level or a low level.
For cooling the speed controller, the speed controller is usually placed in an air flow duct of the air conditioning device through which cool air flows. Thus, it is desired to reduce the size of the speed controller as small as possible for obtaining a satisfactory air flow in the air flow duct.
Hitherto, for reducing the size and weight of the speed controller, there has been proposed a compact resistor unit called "flat resistance" which generally comprises a flat insulating base board and a resistor pattern printed on the base board. The compact unit having such a flat resistance type is shown in, for example, Japanese Utility Model First Provisional Publications Nos. 1-125708 & 2-145507.
Furthermore, at present, there has been proposed a very compact light weight resistor unit, which is schematically shown in FIG. 8 of the accompanying drawings.
As shown in the drawing, the resistor unit comprises a flat resistor 1, two flat insulating plates 2a and 2b which are placed in intimate contact against the respective opposite surfaces of the flat resistor 1, and two flat heat radiation plates 3a and 3b which are placed in intimate contact against the outer surfaces of the two flat insulating plates 2a and 2b. With the parts thus united, a resistor proper part 4 is created.
The resistor block 4 is mounted to a terminal-mounted plastic holder block 6. That is, the holder block 6 has a plurality of terminals 5 of metal mounted thereto. The holder block 6 comprises a rectangular coupler portion 7 and an elongate flange portion 8 on which the terminals 5 are aligned. Each terminal 5 has a lower part embedded in the plastic holder block 6. A so-called "insert molding technique" is used for producing the terminal-mounted plastic holder block 6.
The flange portion 8 is integrally formed at both ends thereof with connecting lugs 9a and 9b through which the resistor block 4 is secured to the holder block 6 with the aid of rivets 10. That is, each rivet 10 extending between the heat radiation plates 3a and 3b is tightly received in an opening formed in the connecting lug 9a or 9b. Although not shown in the drawing, upper portions of the heat radiation plates 3a and 3b are connected through other rivets. Designated by numeral 11 is a projected portion in which a fuse (not shown) is installed.
However, due to inherent construction, the above-mentioned resistor unit has failed to achieve a satisfied durability against shocks applied thereto. That is, because the connecting lugs 9a and 9b are constructed of plastic which is poor in durability, it tends to occur that the lugs 9a and 9b are broken or at least damaged when a strong shock is applied thereto. In fact, such breakage tends to occur when the rivets 10 are brought into engagement with the lugs 9a and 9b for fixing the resistor block 4 to the holder block 6. Such breakage becomes much severe when the plastic lugs 9a and 9b are deteriorated due to long usage of the resistor unit. Furthermore, the plastic lugs 9a and 9b have poor dimensional stability, which tends to induce a loose assembly of the resistor unit.
Furthermore, hitherto, the arrangement of a fuse for such resistor unit has been given little consideration. In fact, in a conventional arrangement of a fuse, there is a possibility that a burnt out part of the fuse causes a short-circuit of two parts. That is, when the fuse is actually operated, the burnt out part of the fuse tends to dangle from a proper position, thereby increasing the possibility of such undesired short-circuit. In fact, the burnt out part of fuse tends to induce various problems in the circuit.